Combustion misfires lead to an increase of the toxic substances emitted during operation of the internal combustion engine and can, furthermore, lead to damage of a catalytic converter in the exhaust-gas system of the engine. It is necessary to detect combustion misfires in the entire rpm and load ranges in order to satisfy statutory requirements for on-board monitoring of exhaust-gas relevant functions. In this context, it is known that, for operation with combustion misfires, characteristic changes of the rpm trace of the engine occur with respect to the normal operation without misfires. Normal operation without misfires and operation with misfires can be distinguished from a comparison of these rpm traces.
A method operating on this basis is disclosed in German patent publication 4,138,765 which corresponds to U.S. patent application Ser. No. 07/818,884, filed Jan. 10, 1992, now abandoned.
According to this known method, each cylinder is assigned a crankshaft angular region which is characterized as a segment. The segments are realized, for example, by markings on a transducer wheel coupled to the crankshaft. The segment time is that time in which the crankshaft passes through this angular region. The segment time is dependent, inter alia, on the energy converted in the combustion stroke. Misfires lead to an increase of the segment times detected in synchronism with ignition. According to the known method, a measure for the rough running of the engine is computed from the differences of the segment times. Additionally, slow dynamic operations can be computer compensated. Such dynamic operations are, for example, the increase of the engine rpm during acceleration of the vehicle. A rough-running value, which is computed for each ignition, is likewise compared to a predetermined threshold value in synchronism with ignition. Exceeding this threshold value is evaluated as a misfire. The threshold value is dependent, as required, from operating parameters such as load and rpm.
The reliability with which the misfires are detected in accordance with this method drops perforce by an amount the fewer individual misfires operate upon the rpm of the crankshaft. The reliability of misfire detection therefore drops with an increase in the number of cylinders of the engine and with an increase of rpm as well as with reducing load.